The tracking microscope is being used to characterize photoresponses due to rhodopsin and to at least one other pigment in the bacterium Halobacterium halobium. The method involves computation of the correlation between the intensity of the stimulus varied as Gaussian white noise and the number of reversals in direction made by an individual bacterium in a given interval of time. Mutants are being isolated and characterized which may have altered rhodopsins, altered receptor membranes, or defects involving the coupling of the receptors to the flagella. Analysis of the mutants is expected to reveal the sequence of events leading from the excitation of the rhodopsin to the control of the flagella, and to an understanding of their molecular bases.